Petrographic and geochemical investigation of magma reservoir processes at Rabaul Caldera, Papua New Guinea

Abstract

Rabaul is a large caldera system located at the northeastern tip of the island of New Britain, Papua New Guinea. Historically, it has been the most active volcano in Papua New Guinea. Rabaul is capable of large volume, high intensity caldera-forming eruptions (high-risk, low-probability events occurring every ca. 2 ka, most recently the Rabaul Pyroclastics event ca. 1400 years ago) and lower intensity but more frequent intra caldera eruptions. All eruptions at Rabaul pose a significant risk to nearby populations and settlements [1,2]. Rabaul’s volcanic rocks vary in composition from basaltic andesite to rhyolite and mostly lie on a common liquid line of descent, controlled by fractional crystallization under reducing conditions [1,2,3,4]. Magma mixing and mingling are common, evidenced by basaltic enclaves and mafic minerals in andesitic and dacitic eruption products [1,2,4]. These enclaves signify regular mafic recharge of the main dacitic reservoir, which primes the system for eruptions [4]. Seismic tomography imaging suggests the presence of an extensive, tabular magma body at a depth of 3-6 km with a volume of about 15-150 km3 [5,6]. The main goal of this study is to determine how the petrology and geochemistry vary between magmatic products erupted from different intervals in the caldera cycle of Rabaul volcano. In addition, we aim to get a better understanding of the primitive magmas that feed and sustain volcanism at the Rabaul Caldera Complex. Furthermore, we identify the key magma chamber processes influencing mineral chemistry and texture. Here, we present EPMA mineral analysis and thermobarometry model outputs for products of the 1937, 1994 and 2014 eruptions as well as for the most mafic eruption products known, the Kombiu basalts. Our thermobarometry data complement existing geophysical observations of the structure of Rabaul’s magma plumbing system. Furthermore, we present SEM imagery to reveal how mineral textures (zoning, resorption, grain size, grain boundaries) in samples from different eruptions at Rabaul reflect magma chamber processes such as mafic recharge, magma mixing and crystallisation.